An international team of researchers - including experts at the Canadian Institute for Theoretical Astrophysics (CITA) in the University of Toronto's Faculty of Arts & Science - has uncovered evidence of a rare type of exploding star, shedding light on one of the most cataclysmic phenomena in the universe.
The study, published in Nature , confirms that black holes in the "forbidden range" - with masses greater than 45 times that of the sun - result from previous black hole mergers, rather than from the collapse of exceptionally massive dying stars.
Using data from the LIGO-Virgo-KAGRA observatory network, the researchers were able to measure the properties of black holes in the forbidden range to confirm the theoretical prediction of pair-instability supernovae - explosions so intense that they destroy the star, leaving no core behind that can collapse into a black hole.
Project collaborator Maya Fishbach, an assistant professor at CITA, said the study highlights the potential of gravitational waves to probe the lives, deaths and afterlives of the most massive stars in our universe.
"We are seeing indirect evidence of one of the most titanic blasts in the cosmos: pair-instability supernovae," Fishbach said. "At the same time, we are finding that once they are born, black holes can grow via repeated mergers."
At the end of their lives, most massive stars collapse into black holes - objects with gravity so strong that not even light can escape. Presumably, the universe's most massive stars should collapse to form the most massive black holes. However, some very massive stars become so hot that they are blown apart in a pair-instability supernova.
The study's first author Hui Tong, a PhD candidate at Monash University in Australia, said the team's analysis of gravitational wave signals - the ripples in the fabric of space-time - detected by the LIGO-Virgo-KAGRA observatory afforded evidence of the existence of the forbidden mass range where stars seemingly don't make black holes.
"The observation is well explained by pair instability; there are no stellar-origin black holes in the forbidden zone because stars are undergoing pair-instability supernovae," said Tong. "The only black holes in this mass range are made from merging smaller black holes, rather than directly from stars."
Confirming the existence of this gap would help settle a major question about how the most massive stars live and die, and the origin of black holes.
First predicted in the 1960s, pair-instability supernovae are challenging to distinguish from more common stellar explosions that leave behind black holes. "We rarely (if ever) get to see these types of explosions in real-time, so it's amazing that we can observe their lasting imprints so clearly in the gravitational-wave data," said co-author Amanda Farah, a CITA postdoctoral fellow. "These imprints - the black holes that pair-instability supernovae fail to leave behind - are already teaching us about nuclear physics. This was one of the early promises of gravitational-wave astronomy, and it's so exciting to see that promise fulfilled today."
Another CITA postdoctoral fellow, co-author Aditya Vijaykumar, said the findings mark an important step forward in gravitational wave research.
"The impact of this work is already being felt across the community. It has already sparked a wave of follow-up research, and there are now multiple lines of evidence suggesting that some black hole mergers involve components born in earlier collisions," Vijaykumar said.
"Such black hole mergers are thought to be produced in regions of the universe that host a dense crowd of stars, and I am excited for all that we will learn about these environments in the future."
